This invention relates to a private branch exchange for use in exchanging information among terminals such as telephones and, in particular, a control data transmission system in such private branch exchange.
Recently, a private branch exchange (PBX) is utilized to allow a call to be made between a telephone and an office line (public communication network) and between telephones in an office. A distribution-controlled PBX is known as one form of PBXs. In this PBX, a common control module and a plurality of line/trunk modules (shelves) are packed into a casing and the respective line/trunk module has a plurality of line/trunk cards to each of which are connected a plurality of telephones or data terminals. The common control module comprises local central processor unit (LCPU) cards corresponding to the respective line/trunk modules, main control processor unit (MCPU) card, time switch (TSW) card, etc. The line/trunk cards in the respective line/trunk module are connected via a control data highway to a corresponding LCPU card in the common control module and via a PCM highway to the TSW card. The TSW card is used for switching voice information transmitted over PCM highways from the line/trunk cards. The respective LCPU performs a control data communication with the line/trunk cards in the corresponding line/trunk module via the control data highway. The line/trunk card is provided for each type of terminals, such as standard telephones and digital telephones.
Recently, the advance in telephone technology requires the attainment of more functions and it is unavoidably accompanied by an increase in the amount of program in the common control module, in order to cope with the function of the terminals. This means an increase in a burden of processing on the common control module. For this reason, hardware, such as a central processor unit and random access memory (RAM), is needed on the line/trunk card to absorb different kinds of processing due to a difference in the type of terminals. It is preferred that, in order to flexibly deal with a change in the contents of processing as well as the demand for extended function of terminals, control data and processing program be down-loaded into RAMs of the respective line/trunk cards from the common control module. In this case, the loading of the program into each of the line/trunk cards leads to an increase in processing time as well as a decrease in system throughput.